The present invention relates to an injection mold for producing a spool. More particularly, the present invention relates to an injection mold for producing a spool used in a photographic film cartridge or cassette for containing photographic film.
As roll-film for photography, 120 and 220 format roll-films are publicly known, in which photographic film with opaque paper is wound on the shaft portion of a spool having flanges at both ends thereof. 120 format roll-film for photography is photographic film having opaque paper fixed to the base side thereof as backing paper. The photographic film, together with the backing paper, is wound on the shaft portion of a spool. In 220 format roll-film for photography, opaque paper as backing paper is omitted, but instead, opaque paper known as "leader paper" is fixed to the leading end of photographic film, and opaque paper known as "trailer" is fixed to the trailing end of the photographic film, thereby allowing the length of the photographic film to be longer and making the number of exposures larger than in the case of 120 format roll-film. In either of the 120 and 220 format roll-films, the width of the opaque paper is set equal to or slightly longer than the distance between the flanges of the spool in order to ensure light-tightness between each side edge of the opaque paper and the inner wall of the associated flange.
FIG. 4 shows the external appearance of 120 format roll-film 1 for photography. As shown in FIG. 5, 120 format photographic film 2 used in the roll-film 1 has opaque paper 3 fixed to the base side thereof as backing paper by using adhesive tape 4. The leading end portion 3a and trailing end portion 3b of the opaque paper 3 each have an end with a narrowed width.
FIG. 6 is a perspective view of a spool 5. FIG. 7 is a sectional view of the spool 5. As shown in these figures, the spool 5 has a shaft portion 6. The shaft portion 6 has a pair of flanges 7 at both ends thereof to regulate two side edges of the opaque paper 3. In addition, the shaft portion 6 has an axially elongate slit 8 in a central portion thereof.
To wind the roll-film 1 on the spool 5, the trailing end portion 3b of the opaque paper 3 is inserted into the slit 8 of the shaft portion 6, and the photographic film 2, which is backed with the opaque paper 3, is wound around the shaft portion 6 with the emulsion side inside. The leading end portion 3a of the opaque paper 3 is folded with a predetermined width and fixed by using adhesive tape 9. It should be noted that each flange 7 has a chucking groove 10 in the center thereof. The chucking groove 10 is engageable with a key shaft of a camera.
There must not be a gap between each side edge of the opaque paper 3 and the inner wall surface 7a of the associated flange 7. Therefore, the width D.sub.1 of the opaque paper 3 is set equal to or slightly wider than the distance (spool inside dimension) D.sub.2 between the inner wall surfaces 7a. When the width D.sub.1 is wider than the spool inside dimension D.sub.2, or when there are variations in the width D.sub.1, the opaque paper 3 cannot be wound fast to the shaft portion 6. As a result, the roll diameter of the roll-film 1 wound on the shaft portion 6 increases undesirably. To prevent this problem, the inner wall surface 7a of each flange 7 is provided with an annular groove 11 (see FIG. 8) with a depth h. The groove 11 is concentric with respect to the shaft portion 6. The groove 11 has a flat trapezoid-shaped or arcuate cross-sectional configuration. Therefore, it is possible to prevent the occurrence of such a problem that the side edges of the opaque paper 3, which are brought into contact with the respective inner wall surfaces 7a of the flanges 7, are undesirably folded and tucked in. Thus, the opaque paper 3 is prevented from causing an undesired increase in the roll diameter. In addition, the side edges of the trailing end portion 3b of the wound opaque paper 3 are completely brought into close contact with the respective inner wall surfaces 7a of the flanges 7. Therefore, the required light-tightness is maintained satisfactorily.
Spools for photographic film such as the above-described spool 5 are usually produced by injection molding process. FIG. 9 is a sectional view of a conventional mold assembled for the injection molding process. A product part (mold cavity) 20 is formed essentially by four mold members 12 to 14. That is, the mold has a stationary mold member 12 for molding the lower half of the outer periphery of the shaft portion 6 of the spool 5 and the lower half of the inner wall surface 7a of each flange 7, and a movable mold member 13 for molding the upper half of the outer periphery of the shaft portion 6 and the upper half of the inner wall surface 7a of each flange 7. The mold further has a pair of slide mold members 14 for molding the outer wall surfaces of the flanges 7.
FIG. 10 is an enlarged view showing a part of the product part 20 in the mold shown in FIG. 9 that corresponds to one flange 7. A movable pin 15 is rigidly provided in the center of each of the slide mold members 14 to form a chucking groove 10 at each end of the shaft portion 6 of the spool 5. In addition, each slide mold member 14 is provided with a cooling water channel 16 for the purpose of shortening the cooling time needed to cool the slide mold member 14.
The conventional injection mold for producing a spool for photographic film is made of a material having high mechanical strength and low thermal conductivity, e.g. a stainless mold steel, exclusive of the movable pins 15. The movable pins 15 are formed by using a material having high thermal conductivity, e.g. a beryllium-copper alloy, with a view to accelerating cooling to thereby facilitate removal.
If the inner wall surface 7a of each flange 7 of the spool 5 to be injection-molded has a groove 11 that is concentric with respect to the shaft portion 6 as shown in FIG. 8, that is, if an undercut 19 is present inside a portion of the product part 20 that corresponds to each flange 7, when the molded spool 5 is removed from the mold, both end surfaces of each of the stationary and movable mold members 12 and 13 must be forcedly removed from the inner wall surfaces 7a of the flanges 7. This may flaw the inner wall surfaces 7a of the flanges 7 of the molded spool 5, causing the light-tightness to be degraded. Furthermore, regarding the shape of the molded spool 5, the flanges 7 tend to bow inward. Therefore, the dimension between the flange inner wall surfaces varies according to the position in the circumferential direction, and thus the flange inner wall surfaces suffer distortion. Furthermore, in order to obtain the dimensional stability of the flanges 7, sufficiently long cooling time is needed. Therefore, it has heretofore been difficult to shorten the molding cycle.